Field of the Invention
The present invention relates to a common mode filter, and more particularly relates to a winding structure of a common mode filter.
Description of Related Art
A common mode filter that is configured by two inductances which is provided on each of two signal lines constituting a transmission path using a differential transmission method, respectively, and magnetically coupled with each other is known. By inserting the common mode filter into the transmission path using a differential transmission method, it is possible to selectively remove only a common-mode noise current.
It is known that a toroidal core or a drum core is used as a specific structure of the common mode filter. The using of the toroidal core makes it possible to obtain high noise-removal performance because no gap exists in the core and it has high effective magnetic permeability. However, the toroidal core has a problem that variation in characteristics is big because automatic coil winding is not applicable and manual coil winding is inevitably required. In contrast to this, the using of the drum core makes it possible to lessen variations in characteristics because an automatic coil winding method can be used. However, the drum core has a problem that it is difficult to obtain as high noise-removal performance as that of the toroidal core. In addition, a drum-core type common mode filter is suitable for mass production because the automatic coil winding method can be utilized.
Each of Japanese Patent Nos. 4789076 and 3973028 discloses an example of a common mode filter configured by using a drum core. In the example of Japanese Patent No. 4789076, two wires each of which constitutes an inductance are wound with a double-layer structure. In contrast, in the example of Japanese Patent No. 3973028, two wires each of which constitutes an inductance are wound together as a pair of wires. Generally, the former winding method is referred to as “layer winding”, and the latter winding method is referred to as “bifilar winding”. Furthermore, Japanese Patent No. 4737268 discloses an example of an automatic coil winder that is used to wind a wire around a drum core.
In recent years, Ethernet has been widely adopted as an in-vehicle LAN. A common mode filter used in in-vehicle Ethernet is required to have more stable characteristics and higher noise-reduction performance than ever before. In this respect, a drum-core type common mode filter has a feature of being able to lessen variations in its characteristics, as described above. Therefore, when noise-reduction performance of the drum-core type common mode filter can be improved, it is possible to obtain the optimized common mode filter for in-vehicle Ethernet.
What is specifically required as high noise-reduction performance is reduction in mode conversion characteristics (Scd) which indicate the rate of a differential signal component, input to a common mode filter, to be converted into a common mode noise and to be output. As a result of extensive studies by the present inventors in order to satisfy the requirement, it has been found that a balance of capacitances caused between different turns of a pair of wires (hereinafter, “capacitance between different turns”) is closely associated with the reduction in the mode conversion characteristics in a common mode filter. Also, high inductance value is required, and then it is expedient to increase the number of turns of the coil for that purpose.